Wohllebe, A. (Corresponding author)FZJ*

1998
Forschungszentrum Jülich GmbH Zentralbibliothek, Verlag Jülich

Report No.: Juel-3513

Abstract: Due to its enhanced absorption in the near infrared with respect to amorphous Silicon microcrystalline Silicon is a promising material for the preparation of stacked thin film solar cells. The aim of this work is the preparation of micro- or polycrystalline Silicon films at low temperatures on low cost substrates like Corning-Glass by laser crystallization using a Nd-Y AG-laser at excitation wave lengths of 1064 and 532 nm. At 1064 nm a crystallization of amorphous silicon films up to a thickness of 800 nm is possible. The crystallites extend from the substrate interface to the surface of the film and show a lateral extent of several hundred nm as well as a sharp interface to the substrate wh ich are promising properties for photovoltaic applications. No preferential orientation was observed. With respect to the crystallite size and the surface morphology of films with a thickness of 400-500 nm the optimum pulse energy per cm$^{2}$ was found between 200 and 400 mJ. Films crystallized at an excitation wavelength of 1064 nm show an increasing surface roughness in the order of magnitude of the film thickness with increasing pulse energy which can not be observed at $\lambda$=532 nm. This difference is caused by nonlinear selfamplifying effects of the heating of the film at 1064 nm which are of no importance at 532 nm. Computer simulations show an enhanced absorption coefficient of the a-Si-films at the used energy densities which is seven times higher than the linear value. The pulse energy dependence of the crystallite size at $\lambda$=532 nm could be attributed to the melting depth of the film by computer simulations showing a maximum grain size if the film is totally melted. Structured laser crystallization revealed that laser crystallized seed points in an amorphous matrix cause larger grain sizes (up to 2 $\mu$m) in a subsequent thermal annealing step with respect to thermal crystallized silicon without a seed pattern. This enhanced crystallite size is due to preferential nucleation at the laser crystallized spots in combination with an enhanced crystal growth speed at twin boundaries. Epitaxy experiments have shown that PECVD-silicon can be epitaxially grown on CoSi$_{2}$, which is thus promising for an application in thin film solar cells.

1. Institut für Schicht- und Ionentechnik (ISI)